Current sensing circuits are widely used for sensing and for controlling/regulating the current that flows through a pass transistor. The current sensing circuits typically comprise a sense transistor which is controlled using the same control signal as the pass transistor. The sense transistor is typically a scaled down copy of the pass transistor.
In order to ensure that the current through the sense transistor is an accurate indication of the current through the pass transistor, the voltage drop across the sense transistor should match the voltage drop across the pass transistor.